One useful class of amplifiers that shows promise for signal amplification in optical fiber based communication systems is the class of optical fiber amplifiers. An optical fiber amplifier includes an optical fiber having a core that is doped, inter alia, with active ions, typically ions of a rare earth such as erbium. In order to operate such a fiber amplifier at the highest efficiency, it is desirable to confine the active ion doped region to a relatively narrow diameter. As a consequence, there is great interest in fiber amplifiers having very small cores, for example, cores less than about 3 .mu.m in diameter, and correspondingly small mode field diameters (MFDs).
Amplifier fibers are disclosed, for instance, in U.S. Pat. No. 4,923,279. See also co-assigned U.S. patent application Ser. No. 467,699, and a recently filed continuation-in-part application thereof, as well as a co-assigned, concurrently filed patent application entitled "System Comprising Er-Doped Fiber," filed for D. J. DiGiovanni et al. and incorporated herein by reference.
In many potential applications, it is envisioned that optical fiber amplifiers will be spliced (i.e., substantially permanently, optically transmissively connected) to standard communication fibers, which exemplarily have a core diameter of about 8.25 .mu.m and an MFD at 1.55 .mu.m of about 9.9 .mu.m.
A standard method for transmissively connecting two lengths of standard communication fiber, referred to as "fusion splicing," involves butting together the prepared ends of two fibers in the presence of a heat source (e.g., a flame or electric arc) such that the fiber ends melt and coalesce. Fusion splices are subject to optical losses, referred to collectively as "splice loss." Various factors have been identified as contributing to splice loss, including lateral offset of the cores, differences in the optical characteristics of the mating fibers, and changes in the refractive index profile that take place during fusion.
When fibers having widely dissimilar MFDs are joined according to the prior art, the mismatch of the mode fields at the location of the splice can result in high splice loss. One technique for mitigating this contribution to the splice loss is described, for example, by D. B. Mortimore and J. V. Wright, "Low-Loss Joints between Dissimilar Fibres by Tapering Fusion Splices," Electronics Letters, 22 (13 Mar. 1986), pp. 318-319. This tapering technique involves first making a standard fusion splice and then drawing the softened glass in the vicinity of the splice such that the glass becomes constricted, decreasing the diameter of both the cladding and the core in the vicinity of the splice. This tapered region is said to function as a mode transformer that transforms the optical mode field of one fiber to that of the other with low optical loss. A standard communication fiber has reportedly been joined, with a total splice loss of 0.56 dB, to a fiber having a core diameter of 3.8 .mu.m and an MFD of 4.34 .mu.m.
Tapering the joint by drawing the fibers is potentially disadvantageous because, inter alia, certain manufacturing difficulties may attach to that method. That is, when the mating fibers have dissimilar outer cladding diameters (ODs), the fiber having the smaller OD tends to constrict more than the larger-OD fiber, causing an abrupt transition from a relatively highly constricted small fiber into a relatively unconstricted large fiber. This effect tends to defeat the purpose of the taper. A similar effect may occur when the fibers have matched ODs, but because of compositional dissimilarities, one fiber has substantially lower viscosity than the other at the fusion temperature. In such a case, the less viscous fiber may suffer the larger constriction. A further manufacturing difficulty obtains because it is conventional to drive the fiber ends together a small distance past the touching point during fusion. It is possible for a bulge to form in the fiber during that process. Such a bulge is undesirable, but will tend to remain after subsequent drawing of the fibers, because constriction will tend to occur at narrow, rather than thick, portions of the fibers. The avoidance of such bulge formation may entail additional penalties in manufacturing time and cost.
It has been observed that during the formation of fusion splices, the index-altering fiber dopants are capable of diffusion. As a result, the refractive index profiles in the fibers near the splice may be changed, and the splice loss may be affected. This effect is discussed, for example, in J. T. Krause, et al., "Splice Loss of Single-Mode Fiber as Related to Fusion Time, Temperature, and Index Profile Alteration," Journal of Lightwave Technology, LT-4, (July 1986), pp. 837-840.
An alternative approach to fusion splicing of fibers, based on this diffusion effect (and here referred to as "diffusion tapering") was reported by, for example, W. Zell, et al., "Low-Loss Fusion Splicing of PCVD-DFSM Fibers," Journal of Lightwave Technology, LT-5, (September 1987), pp. 1192-1195. The approach of Zell, et al. does not involve drawing the fibers, and thus it does not involve substantially changing the physical dimensions of the fibers. Instead, this approach involves spreading the smaller of the cores of the (not very dissimilar) mating fibers by diffusing the index-raising dopant during an annealing step after the splice is formed. (The index-lowering dopant of the cladding was also found to diffuse during heating.)
Zell, et al. reported that diffusion tapering was effective in reducing the optical loss in a fusion splice between a depressed cladding, single-mode (DCSM) fiber and a dispersion flattened, single-mode (DFSM) fiber having a smaller MFD than the DCSM fiber. Significantly, the heat treatment reported in that work caused the concentrations of germanium and fluorine dopants, respectively, to exhibit diffusion profiles extending axially from the joint. Each diffusion profile decayed from a negative peak (i.e., a concentration minimum relative to the background concentration at the specific radial position at which the profile was measured) at the joint to 10% of the peak height (relative to the background concentration) within about 0.5 mm.
At a wavelength of 1.3 .mu.m, a splice loss of 0.30 dB was achieved by Zell, et al. This splice loss was smaller than the theoretical loss in a step joint between the two fibers, and the difference was attributed to diffusion tapering. However, at a wavelength of 1.55 .mu.m, a somewhat greater loss, 0.35 dB, was observed, and no reduction of loss attributable to diffusion tapering was observed.
In a practical communication system, it is desirable for splices between amplifier fibers and communication fibers to exhibit still smaller losses, e.g., losses smaller than 0.3 dB. The Zell, et al. reference does not disclose a technique that can produce low-loss splices between fibers having drastically different core sizes and MFDs. Indeed, at 1.55 .mu.m, which corresponds approximately to the operating wavelength of erbium amplifiers, Zell, et al. has failed to show any improvement in splice loss by diffusion tapering. Moreover, the improved splice reported there involved a pair of only moderately dissimilar fibers both with relatively large cores, i.e., fibers with respective MFDs of 10.1 .mu.m and 7.6 .mu.m at a wavelength of 1.55 .mu.m. Thus, in particular, Zell, et al. does not suggest the possibility that conventional single mode communication fibers could be spliced, with losses than 0.3 dB, to erbium amplifier fibers having MFDs at 1.55 .mu.m of about 4 .mu.m or less.
A fusion splice made with a combination of drawing and annealing was reported by W. Stieb and J. Schulte, "Fusion Splices with Low Loss between SM-Fibers of Different Types," International Wire & Cable Symposium Proceedings, 1988, pp. 569-575. In this work, a splice having a total loss of 0.1 dB was reported between a standard communication fiber and a dispersion-flattened fiber having an MFD of 6.3 .mu.m. Although that method produced a splice having desirable loss characteristics, it is subject to the manufacturing difficulties described above in connection with splices that are constricted by drawing the fibers. In addition, it should be noted that a further manufacturing difficulty attaches because in order to assure that a taper is reproducibly made by drawing the fibers, the temperature to which the glass is heated is generally limited to a narrow range adapted to produce the optimum viscosities in the fused fiber regions. Such a temperature range, although required to produce an appropriate constriction, may not in general be optimal for producing, in addition, a diffused splice.
Thus, practitioners in the art have until now failed to provide a fusion splice that is substantially free of constrictions and that is capable, for operation at about 1.55 .mu.m, of joining a fiber having an MFD greater than about 6 .mu.m to an amplifier fiber having an MFD less than or equal to about 4 .mu.m with a total splice loss less than 0.3 dB.